#include <fstream>
#include <cmath>

struct position
{
	double x, y;
};

int main()
{
	std::ifstream is("in.txt");
	std::ofstream os("out.txt");
	position first, last, tmp;
	is >> first.x >> first.y;
	is >> last.x >> last.y;
	double square = 0;
	while (!is.eof())
	{
		is >> tmp.x >> tmp.y;
		square += ((last.x - first.x) * (tmp.y - first.y) - (tmp.x - first.x) * (last.y - first.y)) / 2.0;
		last = tmp;
	}
	os << std::fixed << abs(square);
	return 0;
}
